A Head Up Display (HUD) is a means of projecting information directly into a human's visual field. The HUD was pioneered for military aviation and has since been used in other applications. HUDs are typically used in aircraft to provide pilots with information superimposed onto their forward field of view through the aircraft windshield. The information displayed may be data or symbolic images indicative of flight conditions such as the operating condition of the aircraft, environmental information or guidance information for use in directing the aircraft to its destination. These images are presented in overlying fashion on the pilot's field of view so as not to interfere with the pilot's view of the background scene.
A HUD uses an optical combiner to add computer generated symbols to the pilot's forward field of view. The system is designed to place this added imagery at optical infinity so that the pilot does not need to refocus eyes when switching between threat scanning and reading the HUD data. Although HUDs are useful in a variety of applications, there are several problems with conventional HUDs, among which high cost, narrow viewing angles, mechanical constraints, and low contrast of images are of particular concern. Optical combiners present problems and challenges in HUD design. For example, many conventional combiners are either undesirably thick or heavy for most applications, or are thin and undesirably highly curved. The thick combiners often contain a pair of cooperative lens elements, at least one of which includes an embedded spherical surface coated with a spectrally reflecting thin film. The external surfaces of these thick combiners are flat so as to provide an undistorted view of the background scene. Thin combiners, on the other hand, typically employ a pair of spherical external surfaces, one of which carries the spectrally reflecting thin film. Thin combiners thus typically do not provide the necessary undistorted view of the background scene, especially when the combiner is thick enough to be adequately durable.
The combiner of the HUD is a critical component to provide a collimated display image to a pilot for a given Field of View (FOV) at a given design reference point. As noted, the HUD combiner components are generally large in size. The current HUD combiners are typically comprised of two optical components and with reflection holograms or dielectric coatings on some of the surfaces. They are cemented together or co-mounted in the HUD system configuration which induces complicated assembly and high cost to the HUD system.
Often, HUD applications include the use of combiners with a doublet lens. The doublet combiner is composed of a convex front lens and a concave back lens. The convex surface of the convex lens is dichroic or holographically coated for reflecting the HUD format information. Since the optical power of the front lens cancels the optical power of the back lens, there is no optical effect when viewing through the combiner. Current HUD applications use lenses which are symmetrical in both of the x and y axes. The x and y axes in this context are the axes along the width and height of the combiner lenses, while the z axis is the axis along the thickness or depth of the combiner lenses. This kind of combiner requires a complicated relay lens system to compensate for optical aberrations.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.